1. Field of the Invention
The present invention relates to a method of X-ray inspection and an apparatus used for the method and, more particularly, to a method of X-ray inspection for inspecting the mounting condition (connection) of electronic devices such as BGAs (Ball Grid Arrays) and CSPs (Chip Scale Packages) which are small in size and have high densities on boards and the like using X-rays and an apparatus used for the method.
2. Description of the Relevant Art
In recent years, the performance of cellular phones, personal computers, video and audio equipment and the like has been getting remarkably higher. What makes it possible is IC packaging technology, which is at the core of the high performance. The density of packages for mounting IC chips and the speed of signal processing have been getting higher.
In particular, array packages such as BGAs and CSPs, which have recently appeared as a means of enabling an innovative packaging technology, and which are effective in having more terminals, have attracted attention.
However, though array packages such as BGAs are excellent at having more terminals, it is difficult to judge whether the mounting condition of an array package on a printed circuit board is good or bad by an optical or a laser visual inspection, since, on account of the construction thereof, the connecting portion of the package and the printed circuit board is hidden from sight by the package itself when it is mounted on the printed circuit board. In the case of fine-pitch packages, it is difficult to accurately pinpoint the location of defects even by an electrical test.
FIG. 1 is a perspective view diagrammatically showing the state of an example of a BGA 1 seen from the side of terminals. FIG. 2 is a perspective view diagrammatically showing a printed circuit board 2 in a situation where the BGA 1 shown in FIG. 1 is mounted thereon. As is obvious from FIG. 2, it is extremely difficult to judge by their appearance whether the connection condition of solder balls 1a, except for those at the outermost periphery of the BGA 1 and the printed circuit board 2, is good or bad when the BGA 1 is mounted on the printed circuit board 2.
Present exemplary techniques of inspecting the connection condition of an array package such as a BGA and a printed circuit board, include a system wherein various and precise two- or three-dimensional perspective images of the connecting portion, seen from a given direction, are obtained (radiography), and a system wherein sectional images of the connecting portion taken as if it had been sliced on a plane parallel to the main surface of the printed circuit board (so-called transverse sectional images) are obtained (sectional radiography).
Radiography uses an X-ray three-dimensional inspection apparatus, for example. FIG. 3 shows examples of X-ray photographs of the connecting portion taken using the X-ray three-dimensional inspection apparatus for radiography.
As is obvious from FIG. 3, by utilizing the X-ray three-dimensional inspection apparatus, the inner shape, which cannot be observed from the outside, can be observed as perspective images. Therefore, even if the inner shape is complicated, whether the inner condition is good or bad can be judged with fair precision.
However, it is difficult to precisely detect the open state of terminals (solder balls) which users like to inspect most in the mounting of array packages such as BGAs and CSPs, forming the heart of the latest high-density packaging. FIG. 4 diagrammatically shows an example of the open state of solder balls.
By obtaining transverse sectional (horizontal slice) images of the connecting portion at two or more vertical positions thereof using sectional radiography, and measuring and comparing the degrees of shadow, the inner shape thereof can be inspected to some extent.
However, it is essentially difficult to precisely detect open terminals (solder balls) in the situation where an array package such as a BGA or a CSP is mounted on a printed circuit board through slice-shaped sectional images of transverse sectional images, because it means trying to detect an open state, which appear in a direction vertical to the main surface of a printed circuit board from the horizontal direction.
The present invention was achieved in order to solve the above problems. It is an object of the present invention to provide a method of and apparatus for X-ray inspection whereby the condition of electronic equipment, like the mounting condition of electronic devices such as BGAs and CSPs, which are steadily getting smaller and reaching higher densities on boards, and particularly open terminals, can be precisely judged.
As described above, hitherto the connecting portion of an array package, such as a BGA and a printed circuit board, hidden from sight by the package itself, has been inspected using perspective images from horizontal or oblique directions (see FIG. 3) or transverse sectional images. But it is essentially impossible to precisely detect opens terminals (solder balls) by these inspection systems.
The present inventor noticed that open solder balls (see FIG. 4) are problems essentially appearing in a direction vertical to a BGA or the main surface of a printed circuit board, rather than in a direction parallel thereto, and appreciated that the detection of open solder balls can be certainly carried out by obtaining sectional images which are vertical to the main surface of a printed circuit board (so-called vertical sectional images), and not by obtaining sectional images which look as if the printed circuit board had been sliced in a direction parallel to the main surface thereof (transverse sectional images) as before. By finding a method whereby vertical sectional images can be photographed, and developing an apparatus with which the method can be realized, the present invention was completed.
FIGS. 5(a) and 5(b) are diagrams showing vertical sectional images of a connecting portion. FIG. 5(a) shows a case wherein no defective connection exists, and FIG. 5(b) shows a case wherein open solder balls 1a exist.
A method of X-ray inspection according to the present invention, wherein a section of a sample is photographed using X-rays to be inspected, includes arranging an X-ray source to apply X-rays and an X-ray detecting means to detect X-rays facing each other with the sample in between. An X-ray incidence plane in the X-ray detecting means is made parallel to the section. The X-ray detecting means is swung about a straight line on the same plane with the section as the central axis, with the parallel relationship between the X-ray incidence plane and the section maintained, while applying X-rays to the sample from the X-ray source as the X-ray source is rotated about the straight line on the same plane with the section as the axis of rotation in synchronization with the X-ray detecting means. X-rays passing through the sample in the X-ray detecting means are then detected.
In the method of X-ray inspection, by mutually moving the X-ray source and the X-ray detecting means while a uniform geometric relationship between them is maintained on the basis of a section of the sample as the subject, a section which is a base of the movements is in a state where it can be regarded as being fixed. On the other hand, the more distant other portions are from the base of the movements, the larger the deformation of the images thereof becomes. As a result, the images thereof become obscure, so that they cannot be the subject of visual recognition. Accordingly, a vertical sectional image is successfully obtained.
This principle is described below using diagrams in FIGS. 6-8, provided for describing the method of X-ray inspection. FIGS. 6, 7 and 8 show a plan view, a front view, and a side view, respectively. FIG. 9 is a perspective view diagrammatically showing a sample.
In the figures, reference numeral 13 represents a sample, and the sample 13 is placed on a stage 14 (FIG. 7). The diagonally shaded area 13a (FIG. 9) represents a section of the sample 13 that is to be the subject. Points A, B, D, E and F are on the section 13a. The points B, A and D are on the same straight line L1, the points E, A and F are on the same straight line L2, and the straight lines L1 and L2 intersect at right angles. A point K in the sample 13 is not on the section 13a but is located at a distance m from the point B on the section 13a. 
An X-ray source 11 and an X-ray detecting means 12 are arranged so as to face each other with the sample 13 in between. X-rays are emitted from the X-ray source 11 and X-rays passing through the sample 13 are detected in the X-ray detecting means 12.
(A) By making an X-ray incidence plane 12a in the X-ray detecting means 12 parallel to the section 13a in the sample 13, the points A, B, D, E and F on the section 13a are projected at points a, b, d, e and f on the X-ray incidence plane 12a in the X-ray detecting means 12 located at H (FIGS. 6-8), respectively. Here, the point a is the center of the X-ray incidence plane 12a. 
(B) The X-ray detecting means 12 is swung or orbited about the straight line L, as a central axis with the parallel relationship between the X-ray incidence plane 12a and the section 13a maintained. The X-ray source 11 is rotated or pivoted about the straight line L1 as the axis of rotation or pivot axis in synchronization with the X-ray detecting means 12. By this operation, the X-ray source 11 moves to g from G, and the X-ray detecting means 12 moves while staying in a position parallel to itself (swings) to h from H.
(C) The points A, B, D, E and F on the section 13a are projected at points a, b, d, e and f on the X-ray incidence plane 12a in the X-ray detecting means 12 located at h, respectively.
As is obvious from FIGS. 6 and 7, distances r1, r2, r3 and r4 between the point a and the points b, d, e and f in the X-ray incidence plane 12a are not changed by the movements (B). The scale of geometric enlargement of each point A, B, D, E or F on the section 13a to the X-ray incidence plane 12a is uniform, and the below relationship is formed.
The scale of geometric enlargement
=Ga/GA=Gb/GB=Gd/GD=Ge/GE=Gf/GF
=ga/gA=gb/gB=gd/gD=ge/gE=gf/gF
The point K located at a distance m from the section 13a is projected at a point kH on the X-ray incidence plane 12a located at H, and is projected at a point kh on the X-ray incidence plane 12a located at h. A gap of a distance r5 is generated between the positions where the point K is projected during the movements (B). As a result, the image becomes obscure. The picture flows and is not fixed.
In the method of X-ray inspection, a picture which can be obtained from the X-ray detecting means 12 takes the form of the section 13a including the straight line L1 and having a parallel relationship with the X-ray incidence plane 12a. In other words, a sectional image of the section 13a including the straight line L1 which is the axis of rotation of the X-ray source 11 and the central axis of the X-ray detecting means 12 and having a parallel relationship with the X-ray incidence plane 12a can be obtained.
Therefore, when a printed circuit board 2 on which a BGA 1 is mounted (see FIG. 2) is placed on the stage 14 and the condition of mounting of the BGA 1 on the printed circuit board 2 is inspected, a sectional image including the straight line L1 and having a parallel relationship with the X-ray incidence plane 12a, a so-called vertical sectional image (see FIG. 5) can be obtained. Using this image, the detection of open terminals (solder balls) can be certainly carried out, and whether the connection condition in the connecting portion, which is hidden from sight by the package itself, is good or bad can be precisely judged.
The above-described method of X-ray inspection is most effective in obtaining a vertical sectional image of the printed circuit board 2, considering the movements of the X-ray source 11 and the X-ray detecting means 12. However, when the connection condition is inspected, for example, it is also possible to obtain not only a vertical sectional image vertical to the main surface of the printed circuit board 2, but also a sectional image oblique or horizontal to the main surface of the printed circuit board 2 (a transverse sectional image in the horizontal case).
A section that is to be the subject can be any section vertical to a stage on which the sample is placed. By choosing a section which is vertical to the stage as the subject section, a vertical sectional image (see FIG. 5) can be obtained. But the subject section can also be any section out of vertical to the stage on which the sample is placed. By choosing a section out of vertical to the stage, a sectional image oblique or horizontal to the stage can be obtained.
Preferably the straight line set as the central axis and the axis of rotation is vertical to a stage on which the sample is placed. By setting the straight line vertical to the stage, a vertical sectional image can be obtained most effectively.
An X-ray inspection apparatus according to the present invention has an X-ray source for applying X-rays and an X-ray detecting means or device for detecting X-rays arranged so as to face each other with a sample in between. X-rays emitted from the X-ray source and passing through the sample are detected by the X-ray detecting means. An X-ray incidence plane of the X-ray detecting means is arranged so as to be parallel to a prescribed straight line. A swinging means swings the X-ray detecting means about the straight line as a central axis while the X-ray incidence plane is kept facing in the same direction all of the time. A first rotating means rotates the X-ray source about the straight line as an axis of rotation in synchronization with the X-ray detecting means.
The X-ray source and the X-ray detecting means are arranged so as to face each other with the sample as a subject in between them. The X-ray incidence plane is arranged so as to be parallel to the straight line. The X-ray detecting means is swung about the straight line as a central axis while the X-ray incidence plane is kept facing in the same direction all of the time. X-rays are applied to the sample from the X-ray source as the X-ray source is rotated about the straight line as the axis of rotation in synchronization with the X-ray detecting means, and X-rays passing through the sample are detected by the X-ray detecting means.
When the X-ray source and the X-ray detecting means mutually move with a uniform geometric relationship between them maintained on the basis of a plane including the straight line and having a parallel relationship with the X-ray incidence plane, the plane is a base of movement and is in a state where it can be regarded as being fixed. Therefore, a section of the sample on the plane that is the base of movement becomes a subject of visual recognition. The more distant the other portions are from the base portion, the larger the deformation of the images thereof becomes. As a result, the images thereof become obscure, so that they cannot be subjects of visual recognition. A sectional image of a section of the sample including the straight line and having a parallel relationship with the X-ray incidence plane can thus be obtained. Accordingly, when the mounting condition of a BGA 1 on a printed circuit board 2 on which the BGA 1 is mounted (see FIG. 2) is inspected, a sectional image of the printed circuit board 2 including the straight line and having a parallel relationship with the X-ray incidence plane can be obtained.
A subject section of the sample is on the same plane with a plane including the straight line, and has a parallel relationship with the X-ray incidence plane. The section can preferably be vertical to a stage on which the sample is placed in the X-ray inspection apparatus.
Because a subject section is any section vertical to the stage, a vertical sectional image (see FIG. 5) can be obtained. Using this resultant image, the detection of open solder balls can be certainly carried out, and whether the connection condition of a package on a printed circuit board, which usually hides inside the package, is good or bad can be precisely judged.
A subject section of the sample, being on the same plane with a plane including the straight line and maintaining a parallel relationship with the X-ray incidence plane, can be out of vertical to the stage on which the sample is placed in the X-ray inspection apparatus.
With a subject section being any section except for those vertical to the stage, a sectional image oblique or horizontal to the stage can be obtained. A horizontal sectional image is a transverse sectional image.
The straight line that is the central axis and the axis of rotation is not vertical to a stage on which the sample is placed in the X-ray inspection apparatus. Because the straight line is vertical to the stage, a vertical sectional image can be obtained most effectively.
A sliding mechanism can be provided for the X-ray detecting means for sliding it in a direction vertical to the X-ray incidence plane. Because the X-ray detecting means can be slid in the direction vertical to the X-ray incidence plane, the position of the sectional image can be finely controlled.
A stage transfer means for two-dimensionally transferring a stage on which the sample is placed can also be provided. Because the stage can be two-dimensionally transferred, a desired sectional image can be easily obtained.
According to another aspect of the invention, a second rotating means is provided to rotate the X-ray source about a prescribed straight line as the axis of rotation. A plurality of the X-ray detecting means are arranged with each of the X-ray incidence planes being positioned so as to be able to form a uniform geometric relationship with the rotating X-ray source on the basis of a prescribed plane including the straight line. Because each of the X-ray detecting means is positioned so as to be able to form a uniform geometric relationship with the rotating X-ray source on the basis of a prescribed plane including the straight line, the plane, as a base of movement, is in a state where it can be regarded as being fixed. Therefore, a section of the sample on the base plane can become a subject of visual recognition. The more distant the other portions are from the base, the larger the deformation of the images thereof becomes. As a result, the images thereof become obscure, so that they cannot be subjects of visual recognition. A sectional image of a prescribed section including the straight line can be obtained without transferring the X-ray detecting means.
Accordingly, when the condition of mounting of a BGA on a printed circuit board on which the BGA is mounted (see FIG. 2) is inspected, a sectional image with respect to a prescribed plane including the straight line, which includes the printed circuit board and the BGA, can be obtained.